The performance or accuracy of a CNC (computer numerical control) machine tool or a coordinate measuring machine (CMM) is determined by the linear displacement errors, straightness errors, squareness errors, angular errors and non-rigid body errors of the machine tool spindle movement (to generalize the moving body which could be something other than a machine tool spindle the term xe2x80x9cbodyxe2x80x9d will be often used instead of spindle). A complete measurement of these errors is very complex and time consuming. Diagonal measurements of the body movement taken continuously along the diagonals of a volumetric space have been recommended for a quick check on the volumetric performance of the machine. However, with this measurement system there is not enough information to identify the error sources for accurate machine quality assessment and error correction.
The characterization of a machine tool body movement is very complex. The machine tool body movement errors are referred to broadly as linear error, angular errors, and squareness errors. The linear errors break down into what are referred to as linear displacement errors and vertical straightness and horizontal straightness errors. Angular errors include pitch, yaw and roll errors. Thus for each of the X, Y and Z axis of motion, there are 6 recognized errors, 3 linear, and 3 angular errors plus 1 squareness error. Thus, for a 3 axis machine, there are a total of 21 of these error.
For a non-rigid body there are many more errors. Existing methods of measuring these errors for calibration and accuracy evaluation using laser interferometer and other means has been very difficult and time consuming until the present invention was developed.
The machine accuracy can be improved by measuring all the above referred to errors and then compensating these errors, providing that the machine tool body movement is repeatable. The key is how to measure these errors accurately and quickly. The inferior previous methods for measuring these errors include the methods disclosed by G. Zhang, R. Ouyang, B. Lu, R. Hocken, R. Veale, and A. Donmes, in an article entitled xe2x80x9cA displacement method for machine geometry calibrationxe2x80x9d, Annals of the CIRP Vol. 37, No. 1. 1988, pp 515-518, and W. L. Beckwith, Jr. in U.S. Pat. No. 4,939,678, grated Jul. 3, 1990 entitled xe2x80x9cMethod for calibration of coordinate measuring Machinexe2x80x9d. These errors for calibration and accuracy evaluation using laser interferometer and other means has been very difficult and time consuming until the present invention was developed.
One example of a laser interferometers which have been used for measuring linear displacement is for example, model HP 5529, manufactured by Hewlett-Packard, Palo Alto, Calif. The straightness accuracy of the body movement using these prior measurement techniques can be measured, for example, by a laser interferometer using angular optics or a quad-detector as, for example, model MCV-3000, manufactured by Optodyne, Inc., of Compton, Calif. The prior art squareness measurements and these other measurements are carried out by directing a laser beam parallel to a selected diagonal of the space in which the body involved can be moved (referred to hereafter as the volumetric space) and the body is moved intermittently along the diagonal as measurements are taken. This process is duplicated for at least one other diagonal and preferably all four diagonals of this space. These laser interferometer diagonal measurements are recommended in the ASME B5.54 standard (section 5.9.2 in Methods for Performance Evaluation of Computer Numerically Controlled Machining Centers, ASME B5.54-1992, American Society of Mechanical Engineers, New York, N.Y.) for the check of volumetric performance.
These intermittent diagonal measurement techniques have heretofore been assumed to be a quick check of the machine accuracy. This was because the diagonal measurement is sensitive to all the errors. Hence, if the diagonal measurement shows the error is small, good machine accuracy has been assured. On the other hand, if the intermittent diagonal measurement shows the error is large, it has been rather difficult to determine the cause of this large error. As above indicated, these prior art techniques are time consuming, particularly if all of the errors are measured. Thus, they need as many as 18 separate setups and measurements respectively for the displacement, straightness, angular and squareness measurements. This can take two of three days to complete which can result in substantial undesired down-time of the machine involved.
Disclosed here is a new measurement method, referred to herein as a vector measurement method. It can measure all these errors, using a simple and portable laser interferometer or a laser Doppler displacement meter (LDDM), in 4 settings and within a few hours. Also, this method is so simple and easy to carry out that in-house personnel of a typical machine tool shop can readily take the measurements and then compensate for the errors involved.
The present invention involves apparatus and a method of using such apparatus which provides a vector measurement technique which enables the movement error components above described to be determined more accurately and efficiently than the prior art. It does so by directing a beam of reflectable energy in a direction non-parallel to the direction of movement of the body along paths having measuring points. (In contrast, the prior art error measurement techniques moved the body in the same direction, that is, along the beam path.) A reflector reflects the energy back to a measuring apparatus which measures the distance between the apparatus and the reflector at these different measuring points. These reflector distance measurements vary with the body position and they are then compared with predetermined ideal reflector distances measurements for these points on the assumption that the body is moved without error to these points by the computer control system involved. This comparison produces reflector distance error values from which the actual body position error components described above can be computed.
Because the prior art error measurement technique moved the body along the beam path and took its measurements at only points along the path, it cannot directly measure the straightness and other error components in directions perpendicular to the direction of body movement.
As in the prior art, the vector measurement technique of the present invention preferably uses a laser beam reflector and a measuring apparatus which measures the distance between the measuring apparatus and the point on the reflector where the laser beam is reflected. However, the broad aspects of the invention encompass the use of reflectable energy other than laser beam energy. Thus it includes the use of such energy sources as radar frequency or other electromagnetic energy sources or an acoustic energy source with measuring apparatus which measures the distance between the source of such energy and the point where a beam of such energy is reflected by a reflector back to the measuring apparatus.
Also, while it is highly preferred that the body to be moved carries the reflector and the measuring apparatus is stationary and directs a beam of the energy involved along a diagonal (or other less preferred direction) in the two or three dimensional space in which the body is moved, the positions of the apparatus and the reflector could be reversed without deviating from the broader aspects of the invention.
In the preferred invention summary now to be made and in the drawings and the description thereof to follow, a much larger than normal laser beam reflector is carried by the body being moved and a stationary laser measuring apparatus directs the laser beam along a diagonal of a three dimensional space. The reflector is sufficiently large that it will intercept the laser beam even when the body is spaced from and is moved along paths which extend a substantial distance from the diagonal. The body is preferably moved sequentially in groups of three repeated incremental steps along the X, Y and Z axes. The first step in each group begins on a segment of the diagonal involved and the body is directed to move a given distance parallel to the first of the axes involved. The laser measuring apparatus then measures and records the actual distance between the apparatus and the point on the laser beam reflector which reflects the laser beam on the selected diagonal. The next step in each group is to direct the body to move a given distance parallel to one of the other two axes a distance where the body can be returned to the end of the segment involved when the body is moved parallel to the third axis involved if the body is ideally moved without any error components. A similar distance measurement is taken and recorded. The last step in each group is to move the body parallel to the third axis to return it to or near the end of the segment involved. This same xyz axes sequence of movement is then repeated until all of the diagonal segments are traversed.
If the body was moved exactly the desired distance each step of the process as directed by the computer program involved then there is no error in the body movement and the distance measurements along the diagonal will confirm this. However, there is almost always some error in the movement distances which will be reflected in the laser beam reflection point distances. Each error type will have an effect in these laser apparatus measured distances. The present invention then desirably calculates the various error components from these laser apparatus measurements and error compensation tables can then be produced which are used to generate control signals which will cause the X, Y and Z axis motor controllers to move the body more accurately as it performs its assigned function.
As compared to the prior art laser measurement technique which moves the body along the same diagonal along which the laser beam is directed, the present invention collects 3 times more data. Furthermore, the data collected after each incremental movement along a given axis is due only to that movement and so that the error sources involved in each of these movements can be separated.
If backlash errors are to be considered, then after all of the measurements are taken as the body involved is moved in the selected sequence from one end of a selected diagonal to the other, the body is then moved with respect to this diagonal and the above mentioned measurements and computations taken as the body is moved in the reverse direction following the similar paths the body was just moved to each point. This requires, of course, the movement of the body along the X, Y and Z axes in the reverse order.
The steps just described carried out along a first selected diagonal then should be carried out along at least another diagonal using the same axis movement sequence as before when the volumetric space is a greatly elongated one (that when a large aspect ratio is involved). The process described is preferably repeated again for at least one more and preferably for all of the diagonals. Since each set of body diagonal measurements for a given sequence produces 3 sets of data, there are 12 sets of data which are collected when all four diagonals are involved in the measurements made. This produces enough data to determine the 3 displacement errors, 6 straightness errors, and 3 squareness errors. If only two diagonals are involved, there is enough data to determine 3 linear displacement and 3 straightness errors.
Part of this improved sequential diagonal measurement technique based on a single sequential three axis movement and data collection method is disclosed in an article of G. Liotto and C. P. Wang, entitled Laser Doppler Displacement Meter (LDDM) Allows new Diagonal Measurement for Large Aspect Ratio Machine Tool easily and accurately in the Proceedings of LAMDAMAP ""97, University of Huddersfield, Queensgate, Huddersfield, England, Jul. 15-17, 1997. The present application adds a further improvement in that the measurements and calculations above described are made with more than 2 diagonals and more than one axis movement sequence. Thus, if one of the sequences was XYZ, the process described above is repeated for at least one more or two added sequences and preferably for all six sequences. Carrying out the process described for three different sequences (e.g. XYZ, YZX and ZXY) over three diagonals will generate 27 sets of data which is enough to determine the rigid body errors (both linear, angular and squareness errors, a total of 21 errors). If the above method is carried out for all six of the possible sequences (i.e. YZY, YYZ and ZYX in addition to the above) for all four diagonals, there will be generated 48 sets of data is enough to solve all the linear errors, angular errors and some non-rigid body errors, which could be used to determine both rigid and non-rigid body errors. The same vector measurement method can also be applied to non-conventional machines such as 4 or more axes machines, non-orthogonal motion machines, hexapod machines and parallel linkage machines.
As above indicated, the present invention uses laser measurement equipment which uses unusually large laser beam reflectors which will intercept the laser beam directed along a diagonal of the volumetric space involved, despite the fact that the body carrying the reflector moves substantial distances laterally of the diagonal involved. If a comer cube or retroreflector is used, conventionally sized reflectors are too small for this purpose. For large lateral deviations, a flat reflector is preferably used as the laser beam reflector. Another aspect of the present invention involves the manner in which the errors determined by the above described aspects of the invention are used to compensate for the errors computed thereby so that the machine tool involved will operate more accurately. These computed errors are used to form compensation tables which can be incorporated into the X, Y and Z axis motor controllers which have to be specially programmed to use these tables. One aspect of the present invention is to use an interpolator software with all the measured errors to provide error compensation signals for the controller which can therefore be a conventional controller.